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Look at it this way, fuel evaporates faster if you toss some on a hot piece of metal versus a cold piece of metal. Regardless of every other consequence of hot versus cold ambient temperatures and their effects on mileage, at some point fuel will freeze if the temperature is low enough and spontaneously ignite if the temperature is high enough.
In my old VX a hot air intake was easy and it made a 10% difference in mileage, in cold weather, in otherwise identical circumstances. It enabled lean burn over a wider range of operation.
In the 02 Insight a radiator block made a 10 MPG difference in winter driving when temps fell below freezing. One of the large factors between winter and summer driving is the temp of the coolant when it enters the engine. That temp can vary by as much as 80 or more degrees depending on ambient temperature. This is because there is no regulation of coolant temp as it exits the radiator. When its 100 outside the exit temp is about 130. When it is -20 that same temp may only be 40 degrees. New radiators make the difference greater and can actually hurt your mileage, especially in winter. Colder temperatures exacerbate the problem because the thermostat restricts the flow of coolant and that slower coolant loses more heat as it passes through the radiator.
regards
Mech
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